1. Field of the Invention
The present invention relates to a communication device that offers a plurality of layer functions, and particularly to flow control by the device.
2. Description of the Related Art
A communication system generally performs flow control for relief of congestion and prevention of data disposal (see Japanese Patent Laid-Open No. 2004-72569). The flow control temporally accumulates data in a buffer and adjusts the amount of data read out from the buffer, for example.
A communication system that has complex functions such as the W-CDMA and has a protocol configuration having a plurality of layers controls traffic according to a protocol in each layer. For example, a W-CDMA packet service connects a mobile terminal and a server using the TCP/IP. The TCP generally controls the data amount according to a slow start algorithm defined in the RFC2001 in data transmission from a server to a terminal.
A W-CDMA system needs to maintain a reserved band by taking into consideration the TCP/IP flow control to guarantee the communication quality. However, the specifications of TCP slow start partially depend on each application, hence it is difficult to predict how much traffic the W-CDMA system transfers on a network. Therefore, the W-CDMA system maintains more reserved bands than the actual traffic volume by allowing for a margin.
FIG. 1 is a graph showing the relation between a reserved band maintained by a W-CDMA system and the actual traffic volume at a TCP/IP level. Referring to FIG. 1, reserved band 91 maintained by a W-CDMA system is fixed to A. On the contrary, traffic volume 92 in a TCP/IP level significantly rises and falls over time.
Traffic volume 92 in a TCP/IP level exceeds the reserved band in part B in the drawing. The traffic volume rapidly decreases because of the flow control upon the occurrence of congestion, and then slowly recovers based on TCP slow start.
However, the above technique has the following problems.
The data amount controlled according to the TCP slow start is not notified to a protocol that has a layer that is lower than the level of the TCP/IP level. As such, it is difficult for a lower-level layer to recognize traffic using a method other than that by measuring the data amount in the layer. Normally, protocols do not cooperate in the traffic control. As a result, a line is not effectively used in a strict sense.
For example, in the slow start as shown in FIG. 1, it takes time until the traffic volume recovers to a level before congestion detection. Until the recovery, the state continues in which a reserved band is not effectively used.
As a practical method for recognizing the traffic volume, a method might be used that assumes average throughput and the number of users for each service, and that calculates throughput in each layer based on the assumption. However, average throughput and the number of users are different for each network. Therefore, it is difficult to obtain information to assume average throughput and the number of users that are accurate.
Further, a W-CDMA system has many connected classes and maximum rates. Furthermore, connectable rates for infrastructures are far more diverse than a second generation system due to the provision of services such as the HSDPA and the HSUPA. Because of the diversity of services for end users, a plurality of users connected at the same rate and in the same class may execute different applications. In that case, the users at the same rate and in the same class have different traffic profiles.
As described in the above, it is very difficult for a service connected on a communication infrastructure to calculate the average rate and the number of connections for each service.
A W-CDMA system uses ATM (Asynchronous Transfer Mode) in a transport layer. The ATM fee is very high in the European market. Therefore, mobile communication operators bear enormous cost as the ATM fee. As such, it is strongly desired to effectively use physical line resources.
The 3GPP (3rd Generation Partnership Project) has standardized the Transport Bearer Modification function. Correct application of the function serves to establish cooperation between a frame protocol and its lower ATM/AAL2 layer, and hence the physical line resources can be effectively used to a certain extent. However, a technique is desired to use a line more effectively in the above circumstances.